The present invention relates to a combined cycle power generation plant, which cools a high temperature section of a gas turbine plant using a steam as a cooling medium and also relates to an operating method thereof.
In a recent thermal power generation plant, in order to improve a plant heat (thermal) efficiency, many combined cycle power generation plants, each of which is composed of a gas turbine plant, a steam turbine plant and an exhaust heat recovery boiler, are operated as a practical equipment (practically usable plant). In the combined cycle power generation plant, there are provided a so-called single-shaft type plant and a so-called multi-shaft type plant.
The single-shaft type plant is constructed in a manner that one steam turbine is directly connected to one gas turbine through a shaft, and a plurality of shaft systems are provided so as to meet a planned plant power output. On the other hand, the multi-shaft type plant is constructed in a manner that a plurality of shafts of the gas turbines are independently and separately provided with respect: to one steam turbine.
The single-shaft type plant is constructed in a manner that one shaft system and the other shaft system do not interfere with each other, so that, in the case where the output power of the plurality of shaft systems is lowered during a partial load operation, the single-shaft type has an advantage of preventing a plant thermal (heat) efficiency from being rapidly lowered. On the other hand, the above multi-shaft type makes much of the steam turbine, and makes capacity (power output) large, so that the multi-shaft type has an advantage that the plant thermal efficiency during a rating operation becomes high by an increase of the capacity as compared with the single-shaft type.
As described above, since both single-shaft type and multi-shaft type have advantages, both types are operated as a practical equipment.
In both the single-shaft type and multi-shaft type combined cycle power generation plants, in order to economize fuel consumption and to reduce a unit price of power generation, it has been studied and developed to further improve plant thermal efficiency. The plant thermal efficiency is calculated from a ratio of the total sum of heat output of plants such as a gas turbine plant, a steam turbine plant and an exhaust heat recovery boiler to the total sum of heat input thereof. When reviewing the steam turbine plant, the gas turbine plant and the exhaust heat recovery boiler in an attempt to improve the plant thermal efficiency, it can be presumed the the steam turbine plant and the exhaust heat recovery boiler have already reached their efficiency limits. However, it is expected that an improvement in the thermal efficiency of the gas turbine will result in the improvement of a plant thermal efficiency of the entire combined cycle power generation plant.
In the gas turbine plant, the higher an inlet combustion gas temperature of the gas turbine is, the more the thermal efficiency can be improved. Further, through recent development, in heat resistant material and progress in cooling techniques the inlet combustion gas temperature of the gas turbine is increased to 1500.degree. C. or more instead of 1000.degree. C. to 1300.degree. C.
In the case where the inlet combustion gas temperature of the gas turbine is 1500.degree. C. or more, although heat resistant material has been developed, a high temperature section of the gas turbine, for example, a gas turbine stationary blade, a gas turbine moving blade, a liner transition piece of a combustor and the like, has already reached its maximum allowable metal temperature. For this reason, during an operation having many start-up and stopping times and a continuous operation of the plant for a long time, there is a high probability that accidents resulting from material breakage and fusion can happen. Thus, in the case of increasing the inlet combustion gas temperature of the gas turbine, in order to maintain the gas temperature within the allowable metal temperature of components of the high temperature section of the gas turbine, there has been developed a technique of cooling the high temperature section of the gas turbine using an air, and the practical equipment has been already realized.
However, in the case of cooling the high temperature section of the gas turbine using an air, an air compressor connected directly to the gas turbine is used as an air supply source. High pressure air supplied from the air compressor to the gas turbine are used for cooling the high temperature section of the gas turbine. Further, after the high pressure air is used to cool a turbine blade it is discharged as a gas turbine driving gas and, for this reason, the temperature of the gas turbine driving gas lowers, and a mixing loss is caused. This is a factor hindering the improvement of the plant thermal efficiency.
Recently, steam has been reconsidered as a cooling medium for cooling the high temperature section of the gas turbine, for example, the gas turbine stationary blade, a gas turbine moving blade, etc. The technique of using the steam as a cooling medium has been disclosed in a journal of Machinery Society of America (ASME thesis, 92-GT-240) and Japanese Patent Laid-Open Publication No. HEI 5-163961.
The steam has a specific heat valve twice as high as air, and is excellent in heat conductive performance. Further, in the steam, close loop cooling will possibly be adopted without lowering the temperature of the gas turbine driving gas and causing a mixing loss. Thus, it is expected that the steam is applied to a practical equipment in order to contribute to the improvement of plant efficiency.
However, in the case of using steam as a cooling medium for cooling the high temperature section of the gas turbine, there are several problems because the combined cycle power generation plant is a multi-shaft type.
In general, in the case of cooling the high temperature section of the gas turbine with steam, a driving steam of the steam turbine plant has been used.
However, in the multi-shaft type combined cycle power generation plant, a plurality of gas turbine plants are combined with respect to one steam turbine plant. For example, during a partial load operation, when only one of the plurality of gas turbine plants is operated, conditions (temperature, pressure, flow rate) of the steam supplied from the exhaust heat recovery boiler to the steam turbine plant greatly vary from design values. If the high temperature section of the gas turbine is cooled under varied steam conditions, the following problem and disadvantage will be caused.
For example, in the case where the temperature of the steam for cooling the high temperature section of the gas turbine is high, it is difficult to maintain a material strength of the gas turbine moving and stationary blades, a gas turbine rotor or the like and, for this reason, the use of the high temperature steam will be a factor of causing breakage and fusion. On the other hand, in the case where the aforesaid temperature of steam is low, an excessive thermal stress, due to the difference in temperatures between the gas turbine driving gas and a cooling steam, is locally generated in the gas turbine moving and stationary blades, a gas turbine rotor or the like. In the case where the steam temperature is further lower the steam is easy to drain, and a local thermal stress, resulting from excessive cooling, will be generated.
On the other hand, in the case where the pressure of the steam is high, the gas turbine moving and stationary blades are formed to have a thin thickness and, for this reason, there is a possibility that breakage will be caused due to a so-called ballooning (expansion by internal pressure). Further, in the case where the pressure of the steam is low, there is a possibility that the gas turbine driving gas flows into the moving and stationary blades.
Moreover, in the case where the flow rate of steam decreases, the gas turbine rotating and stationary blades, a gas turbine rotor and the like can not perform preferable cooling function and, for this reason, it is impossible to maintain their material strength. Therefore, it is difficult to cope with a high temperature gas turbine plant.
Furthermore, in the case where the temperature and pressure of steam are high, water is used with temperature and pressure reducing devices so that the temperature and pressure of the steam can be adjusted so as to provide a proper temperature and pressure. In this case, if the water is not sufficiently purified, dust gathers in a passage of the gas turbine moving and stationary blades and, for this reason, the passage is jammed with dust. As a result, the cooling performance lowers, which will constitute a factor of oxidation and corrosion.
As described above, in the multi-shaft type combined cycle power generation plant, in the case of cooling the high temperature section of the gas turbine using steam, there exists a close relationship between a fluctuation of conditions of the cooling steam supplied to the high temperature section of the gas turbine from the steam turbine plant and an increase and decrease in the number of operating gas turbine plants. For this reason, no fluctuation needs to be caused in the steam conditions. In particular, during a start-up operation and a partial load operation, it is difficult to adjust the steam conditions of the cooling steam supplied to the high temperature section of the gas turbine within a design value because a study and development of such adjustment has still not been sufficiently made.